Electrical energy storage cells of a flat roll configuration, contained in a casing include electric double-layer capacitors, lithium-ion battery cells, lithium-ion capacitors, and the like.
The electric double-layer capacitor is one such that polarization electrodes—cathode, i.e., positive electrode and anode, i.e., negative electrode—are placed that face to each other by way of a separator, and double-layer capacitance to be formed on surfaces of the polarization electrodes in an electrolytic solution is used.
The lithium-ion battery cell uses oxide compound such as of cobalt, nickel, manganese as a cathode (positive electrode) material, and carbon as an anode (negative electrode) material. The feature thereof is that the lithium can stably be charged and stored in the carbon anode.
In addition, a lithium-ion capacitor has been developed as a new electric double-layer capacitor. The lithium-ion capacitor is one such that lithium ions are doped into the anode of the electric double-layer capacitor, having a characteristic such that the lower limit voltage cannot be decreased to zero volts although an upper limit voltage higher than the voltage of the electric double-layer capacitor is gained.
Because of being contained in a sealed casing such as an aluminum-laminated casing or a metallic casing, such electrical energy storage cells can be configured to be low-cost and compact. However, since each output voltage thereof, which is in the order of 3 to 4 V, is lower when compared with an output voltage of 400 V of an aluminum electrolytic capacitor, typically, an electrical energy storage module is used that is constituted by interconnecting in series a plurality of the storage cells to thereby raise the output voltage.
Electrodes of a flat roll configuration for use in the electrical energy storage cell (hereinafter also called flat roll electrical energy storage cell) are formed in such a manner that the cathode layers and anode layers are applied onto strips of cathode current collector foil and anode current collector foil each having a thickness in the order of 10 to 30 micro-meters, and then such layers and foils are wound into a flat roll configuration, via a strip of porous separator made of cellulose or olefinic resin fiber, by several meters through several tens of meters, with a core of the flat roll configuration being centered. In comparison with a lamination type electrical energy storage cell having several tens of strip electrode laminated, the thus configured flat roll electrical energy storage cell has an advantage in low costs and mass production because of being rolled in a short time. Here, in many cases, roll cores are used only at the time of forming a flat roll configuration, and the cores are drawn out from the cell at the time of its completion. When the cores are left as they are, there is used a hollow tubular member(s) made of metal such as aluminum or of resin, or the like.
It is expected from a viewpoint of energy saving that such flat roll electric double-layer capacitors and lithium-ion capacitors will be applied to storage of regeneration energy generated by a motor. When used for brake regeneration in servo motors, elevator traction machines, or electric railroad cars, and for regeneration of electric motors of hybrid vehicles or the like, such capacitors are requested to achieve the performance of repetitive charging and discharging for a large current capacity that is more than 100 A. When the large current flows, power loss is generated in proportion to internal resistance and the square of the current, thereby reducing efficiency in the charging and discharging. Moreover, because of the power loss automatically resulting in heat generation, unless the current collector foil, which is a thermal conductor, rapidly carries out heat dissipation, then the temperature inside the electrical energy storage cell increases, which leads to sharp reduction in performance, thus causing reduction in life span. In particular, during charging, since the electrode swells and allows electrolyte to be taken in, it is likely to cause a deficiency of the electrolyte, which has been a factor that accelerates degradation. For that reason, to achieve the rapid charging and discharging performance in the flat roll electrical power storage cell, it has been necessary that the current collector resistance be significantly reduced to avoid heat build-up in the cell, as well as significantly reducing heat dissipation from the current collector foil. Note that the current collector resistance refers to an electric resistance from current collector foils of the cathode and anode until reaching respective current terminals.
In general, conventional flat roll electrical energy storage cells are constructed in which metallic tubs are ultrasonically bonded at a several places of the current collector foil, and are connected to current terminals for drawing out of the casing. Further, they are configured so that the electrolyte permeates, along the axial, into the separator by making the length of the separator in the axial direction longer than that of the current collector foil. For instance, the configuration of a typical flat roll lithium-ion battery cell is disclosed in which the separator stretches out to the outside in the axial direction, and tubs attached to the electrodes serve as the current terminal (e.g., Japanese Unexamined Patent Publication 2007-265989 (page 7, FIG. 3)).
Another configuration in a different flat roll electrical energy storage cell is disclosed in which the cathodes (positive electrodes) and anodes (negative electrodes) are each provided with an edge portion where no electrode layers oppositely extending away from the separator along the roll core axis are applied, and each cathode and anode terminals are bonded to the edge portion (e.g., Japanese Unexamined Patent Publication 2000-40501 (page 2, FIG. 1)).
Because in the construction in which metallic tubs are ultrasonically bonded at a several places of the current collector foil and are connected to current terminals for drawing out of the casing, only limited areas of the metallic tubs are bonded to the current collector foil, a problem has been that the heat generated from the current collector foil cannot efficiently be dissipated, as well as a problem of the current collector resistance that cannot be reduced. Another problem has been that the heat generated from the current collector foil cannot efficiently be dissipated because the thermal conductivity of the separator is lower than that of the current collector foil in the construction in which the separator stretches out to the outside in the axial direction. Furthermore, in the configuration in which edge portions are provided where no electrode layers oppositely projecting from the separator along the roll core axis are applied and in which the edge portions are connected to the edge portion, still another problem has been that although heat dissipation and interconnection resistance are reduced, the electrolyte becomes difficult to migrate, resulting in a deficiency of the electrolyte in some part of the internal separator, which hastens degradation of the separator.